A conventional hydraulic disc brake for a bicycle includes a caliper having a pair of opposing cylinders each receiving a piston having a brake pad operatively associated therewith. The opposing pistons are hydraulically actuated to close upon and squeeze a disc between the opposing brake pads. Typically, both pistons are driven through a single hydraulic hose by a master cylinder. The vast majority of bicycle hydraulic disc brakes include a caliper that is fixedly attached to the bicycle frame and a flexible disc. The disc is flexible to minimize its weight, an important attribute for bicycle components.
It is desired that as the master cylinder is pressurized opposing cylinders of the caliper containing the pistons are pressurized at the same time and an equal volume of hydraulic fluid is provided to each piston so that each piston advances equally to simultaneously bring the associated brake pads into contact with the flexible disc and to also retract equally to provide clearance between the brake pads and the disc with the master cylinder unpressurized. In practice, it is not uncommon for one of the pistons to experience some interference which inhibits advancement of the piston as the master cylinder pressurizes the hydraulic system. While not intending to provide an exhaustive list of the causes of this interference, interference may be caused by the following factors: non-conformity with the tolerances of a seal associated with the piston; non-conformity with the tolerances of the piston; non-conformity with the tolerances of a groove that receives the seal in a caliper cylinder; irregularities in the surface of the piston or the seal resulting in variations in the coefficient of friction therebetween; and contamination such as dirt or grit may invade the surfaces between the piston and the seal. The factors can also combine, resulting in tolerance stack up. Because of “hydraulic cross-over”, which is the tendency of a greater volume of hydraulic fluid from the single hydraulic line from the master cylinder to flow to drive a less restricted piston of the caliper piston pair, the less restricted piston will advance more readily than a more restricted piston, which will result in some deflection of the flexible disc by the brake pad associated with the less restricted piston (the “first contacting brake pad”) until such time as the more restricted piston overcomes the interference and releases or the first contacting brake pad simply drives the flexible disc into contact with the other brake pad operatively associated with the more restricted piston or even the caliper body. If the deflection of the disc is enough to cause the less restricted piston to slip forward in the seal beyond the seal retraction, eventually, upon depressurization of the master cylinder, the pad of the less restricted piston will remain in contact with the disc, causing unnecessary friction and perhaps a bothersome noise to the rider.
Conventional bicycle disc brakes have evolved from motor vehicle disc brake systems and are essentially miniaturized versions thereof with few operational modifications. The problem of a brake pad rubbing a disc is not a significant problem for motor vehicle disc brakes, primarily because a motor vehicle has ample driving power from the motor to overcome the attendant frictional losses and any noise generated by the contact is relatively far away from motor vehicle operator and furthermore, any noise is masked by the operational noise of the vehicle. In addition, motor vehicles typically have comparatively rigid discs. As used herein, a “rigid disc” means a disc that does not flex enough when contacted by a pad of a less restricted piston to allow the less restricted piston to slip forward without the other piston releasing.
The present disclosure is intended to overcome one or more of the problems discussed above.